Texturing models in Blender
I believe some of your face normals are flipped OR you have overlapping faces / really rough topology, OR you need to unwrap your model more cleanly without UV distortion and maybe even increase the UV texture resolution.

To fix normals: Tab for Edit Mode, then A to select all. Ctrl+Shift+N to recalculate normals (untick "inside").

Fix overlapping geometry: a quick fix might be Re-Mesh modifier. Be sure to save an alternate version before flattening ("applying") a modifier because it is destructive.

UV Unwrap quick fix: Edit Mode, select all faces, U "Lightmap Pack" or "Smart UV Project". Make sure your UV image texture is a large enough resolution and that your UV islands have enough margins between to prevent overlap.